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So, let's say that Humanity goes interplanetary. Being humans, we start fighting amongst ourselves on other planets. The Moon, Mars, and wherever else we decide to go. So, naturally, we make more guns. After a decade or two of mass-producing ridiculous amounts of bullets, we run out of gunpowder. Oh well. Humans have a surprising capacity to invent creative ways to kill each-other.

Q: What would be a good alternative to the explosives used in the propulsion of projectiles in an automatic weapon?

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    $\begingroup$ You do realize that sulfur is a significant ingredient in more than simply gunpowder manufacturing, right? If some other technology (gauss guns, or energy weapons, for example) were more effective then it would make sense to replace traditional firearms. But "running our of sulfur" is not only improbable, it is really bad news. $\endgroup$ – AndreiROM Nov 8 '16 at 17:29
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    $\begingroup$ Running out of sulfur is a pretty ridiculous possibility. But, you could have rail guns, or an alternative explosive propellant (there are skads of them), or some variant of compressed air, or kinetic energy stored in a flywheel. $\endgroup$ – ohwilleke Nov 8 '16 at 17:33
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    $\begingroup$ @AndreiROM I'd say that is out of scope, the OP is asking for alternatives to gunpowder guns, perhaps he already has thought of what the lack of sulfur would cause. $\endgroup$ – TrEs-2b Nov 8 '16 at 17:34
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    $\begingroup$ We do not use gunpowder anymore, so how can we run out of it? Edit your question if it's about all explosives. But note that as long as you have paper and fertilizers available, "running out of" is not feasible. If we can't have flash cotton and similar stuff, it also means education is in ruin and agriculture can't feed even a half of people we have now. $\endgroup$ – Mołot Nov 8 '16 at 23:17
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    $\begingroup$ This constraint is ridiculous on its face. We can propel a 165 ton space shuttle to speeds over 17500 miles an hour by combining oxygen and hydrogen in a big, controlled explosion, but we somehow lose the ability to propel a metal pebble out of a gun barrel with a tiny explosion? Come on. $\endgroup$ – HopelessN00b Nov 9 '16 at 0:05
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Electrical weaponry

The most logical option for a future humanity would be railguns or coilguns, even today electrically powered weaponry is a viable (but still sub-par) choice. It seems to me that gunpowder weaponry is as good as it can get, and almost all upgrades or new innovations to modern weaponry are based around electronic additions or advanced manufacturing techniques.

With our current rate of technological expansion, electricity storage and handling will likely be good enough to surpass modern gunpowder weaponry by the time humanity goes interstellar, and since these guns just run on energy, the production and maintenance of such weapons would be a simple endeavour.

There are two types available, railguns or coilguns. Railguns are very simplistic, as you just feed electricity through the projectile, an on/off switch is all you really need. Railguns are prone to wear however, particularly the point of contact between bullet and rail. Replacement components may be needed if firing at high power and high rates. Also, the rails will try to force each other apart with each shot, requiring a strong design.

Coilguns are more complex, you need multiple coils to activate as the projectile passes through them, requiring more electronics and circuitry, and thus creating more points of failure. Greater speeds can be achieved however, and there may be less wear, as the projectile need not come in contact with the rest of the components, the coils can be wrapped around the barrel and stay separate. Finally, since each coil is separate and doesn't need to touch other components, they can be completely insulated from the outside world, but a railgun requires its high voltage rails to be open to the world, risk of electrocution, keep rain of the barrel!

Pros:

  • Powered by electricity alone

  • Simple to manufacture, pure iron bullets would suffice for the ammo

  • Almost no moving parts, would only need them to load the chamber

Cons:

  • Projectiles must be magnetic, so heavier elements or exotic rounds (incendiary/explosive) may be difficult to make

  • Extra points of failure. Electronics are more fragile than the machined mechanics of current weaponry, each component is likely a point of failure

  • Vulnerable to EMPs, such a tactic may be able to disable an entire army if not properly defended against. Coilguns in particular.

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    $\begingroup$ Just as a brief note, there's not actually any reason that a rail/coil gun projectile couldn't be incendiary or explosive or whatever else as well - put an explosive or incendiary tip on an iron slug, and there you go. $\endgroup$ – HopelessN00b Nov 8 '16 at 23:57
  • $\begingroup$ I don't know much about ammo so I'd love to hear more about this. My assumption is that in modern weaponry, changing the mass of a projectile by replacing metal with compound wouldn't be problematic, the round would simply have a change in muzzle velocity or contained propellant. But with a 'mag-round' removing metal mass for incendiary compound also means removing magnetic capability, and a slower round in a coilgun would be a problem for the timing of the coils. Could this be a legitimate concern? Would a timing-setting or something be required? $\endgroup$ – Bazul Nov 9 '16 at 16:53
  • $\begingroup$ Yeah, the timing would (likely) be slightly different, so you'd likely want some sort of adjustable timing logic in your coil gun, for optimal projectile velocity... assuming they wouldn't have that anyway. We have computers in so much these days, (including advanced small-arms/infantry weapons), it's hard to imagine that future weapons systems like coil guns wouldn't have this ability natively. $\endgroup$ – HopelessN00b Nov 9 '16 at 17:40
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Air rifles come to mind as a good answer, but automatic designs are fairly new. The repeating crossbow was a design used in the ending days of bows, but it is only semi-automatic (though it could become automatic with time).

The rubber Gatling gun is a great option but it lacks lethality, I'd suggest watching the slingshot channel for way to make this more lethal.

And these are only the modern ones! There are projects working of railguns, coilguns, even lightning rifles!

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    $\begingroup$ Uhm ... you know that's just a painted AR-15 with a battery shoved up the magazine well, right? $\endgroup$ – AndreiROM Nov 8 '16 at 18:19
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    $\begingroup$ @AndreiROM I do now, I've been tricked, filthy corporate fat cats, not giving me my automatic nail guns $\endgroup$ – TrEs-2b Nov 8 '16 at 18:28
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    $\begingroup$ I'm sorry to have been the one to burst your bubble. Let us hope that your desperate cries for justice will be heard. $\endgroup$ – AndreiROM Nov 8 '16 at 18:35
  • $\begingroup$ The earliest multi-shot firearms were multi-barreled monstrosities. Early airguns might have evolved in that direction had firearms not been available. $\endgroup$ – Kevin Krumwiede Nov 9 '16 at 5:46
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There are plenty of effective propellants which don't require sulphur. Indeed, 'black powder' has been obsolete for nearly a century in firearms. Just one example is cordite which has been widely used as a propellant in both small arms and artillery.

If for some reason you have no explosive propellants at all (unlikely) compressed gas is the next best thing and there are air rifles at least equivalent to small bore cartridges and even then their development is mostly limited by the legal restrictions and the fact that self contained explosive cartridges are more practical for higher powered weapons.

While many airguns are maually cocked the self loading wepons only require a pressurised gas reservoir and a suitable chambering mechanism. While there are few truly self loading high powered air weapons this has more to do with commercial and legal retraints than any fundamental engineering difficulty.

Similarly an electromagnetic gun is not much more than a solenoid.

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It's impossible to run out of gunpowder while sustaining carbon-nitrogen based life

As long as you have any agriculture whatsoever, you can produce something like nitrocellulose from the same basic components. If we did magically run out of very common base elements, the main impact wouldn't be on the arms industry (which needs comparatively small amounts of it) but on food production instead; and even then it would be possible to produce gunpowder if fueling war is considered more important than feeding poor people - as it has been many times in our history.

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While other answers have already stated air rifles, there is something more to them that increases their importance tenfold in space-gun-battles. This aspect has not been highlighted in the posted answers, so I will only get in some detail with this.

You know that gases like ammonia and carbon dioxide exist in solid/frozen state beyond Mars. This means that in far-away places, compressing gases is no real hassle at all as they are already solid when you fill them in your gigantic battleship cylinders. All you have to do is to heat the up inside your battleship cylinders in orders to turn them into gaseous state, getting your space-guns ready for firing.

This means that airguns in space don't require any special compressed filling station to fill them up. You simply collect some frozen gases from the chilly neighborhood and fill up your reserves. In the reserve, they would still be present in solid state, as you haven't heated them there. The heating mechanism would be present in guns individually which would quickly get them ready for shooting.

So your space airguns would have a safety switch which is actually just an on/off button for the simple heating mechanism for the gas in the cylinder. As long as the safety switch is not pulled, the gun will not function, as the gas is still in solid state and would have zero pressure to propel the bullets. However, once the safety switch is pulled, the solid volatile would be heated into gas and the weapon would be ready for action.

Simple. Cheap. Effective.

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